I got rid of all my tanks except my nanos and I want to put some money into my fluval flora to make it an excellent tank.

I've been using the cartridges but they're a total PITA. I put a diffuser on it and after not using it in months I bombed my tank by starting it up and getting distracted before setting it to an acceptable level.

I have a bubble counter and diffuser in one already. Also have a drop checker.

Whats the cheapest I can get away with while doing the job properly?

I've heard the paintball canisters aren't able to be regulated, is this true? I don't want the same problem as the cartridges.

paintball tanks can be regulated. I used to play paintball there are plenty of paintball regulators out there most bring the pressure down to around 800psi you would need another regulator to bring the pressure down further. Although I'm sure if you searched online someone somewhere has put together a paintball regulator system for aquarium use.